Number reduced, would you believe? (2,4)
I believe the answer is:
no less
'would you believe?' is the definition.
('no less' is said after a surprising claim)
'number reduced' is the wordplay.
'number' becomes 'no' (abbreviation).
'reduced' becomes 'less' (a reduced amount).
'no'+'less'='NO LESS'
(Other definitions for no less that I've seen before include "praise, ironically" , "indeed!" , "Phrase ironically used to express admiration" , "Ironic comment - just as much" , "Bare minimum" .)